1. Field of the Invention
The present invention relates to a safety circuit in a circuit for controlling an AC current flowing in a load by means of a semiconductor control rectifying element such as a thyristor or bidirectional thyristor (triac).
2. Description of the Prior Art
FIG. 2 shows an example of a conventional AC control circuit. As shown in FIG. 2, an AC voltage is applied between terminals 1 and 2 of an AC control circuit 22. A load 3 such as a fixing heater of a fixer and a bidirectional thyristor 4 as a semiconductor control rectifying element are series-connected between the terminals 1 and 2.
A trigger pulse to be applied to a gate G of the bidirectional thyristor 4 is applied between terminals 5 and 6.
A photocoupler 7 comprises an LED 8 and a phototransistor 11. One terminal 7-1 of the photocoupler 7 is connected to the terminal 5 through a current limiting resistor 9. The other terminal 7-2 of the photocoupler 7 is connected to the terminal 6. A diode 10 is reverse-biased between the two ends of the LED 8. When the trigger pulse is applied between the terminals 5 and 6, the LED 8 is turned on. The phototransistor 11 partially constituting the photocoupler 7 is turned on, a transistor 12 is turned on, and a current flows to the gate G of the bidirectional thyristor 4 and turns it on. Resistor 13A and 13B are for flowing a gate current, and a resistor 14 and a capacitor 15 are for noise filtering, i.e. providing a path for noise to be eliminated regardless of the state of the bidirectional thyristor 4.
A varistor 16 serves to prevent flow of a surge current. A resistor 17, a diode 18, and an electrolytic capacitor 19 constitute a half wave rectifying circuit. A Zener diode 20 generates a fixed voltage across the two ends of the electrolytic capacitor 19. A resistor 21 determines a base voltage of the transistor 12.
In the circuit arrangement described above, even if a gate trigger pulse is not applied to the bidirectional thyristor 4, anodes A1 and A2 of the bidirectional thyristor 4 form a short circuit and an AC current may be kept flowing to the load 3. Thus, when the load 3 is a heater, the heater may be overheated or may cause a fire.